Transistors are formed by depositing at least one layer of semiconductor material on a substrate comprising a polyphenylene polyimide. The substrate permits the use of processing temperatures in excess of 300.degree. C. during the processes used to form the transistors, thus allowing the formation of high quality silicon semiconductor layers. The substrate also has a low coefficient of thermal expansion, which closely matches that of silicon, thus reducing any tendency for a silicon layer to crack or delaminate.